Block copolymer composition, process for producing the same, and pressure-sensitive adhesive composition

ABSTRACT

The present invention provides a composition which comprises a radial poly(aromatic vinyl)/polyisoprene block copolymer as a major component and which can be used as a more excellent pressure-sensitive ingredient. This composition is obtained by reacting a diblock polymer with a coupling agent having a functionality of 4 or higher in the presence of a coupling accelerator, and it contains (a) 1 to 34% by weight of a diblock polymer, (b) 34 to 99% by weight of a four-branch polymer, and (c) 0 to 50% by weight of a two-branch polymer and/or a three-branch polymer. The poly(aromatic vinyl)/polyisoprene block copolymer contained in the composition has a weight-average molecular weight (Mw) of 260,000 to 500,000, and the poly(aromatic vinyl)/polyisoprene block copolymer has a poly(aromatic vinyl) block content of 5 to 24% by weight.

TECHNICAL FIELD

This invention relates to poly(aromatic vinyl)/polyisoprene blockcopolymer compositions suitable for pressure-sensitive adhesive use, aprocess for producing the same, and pressure-sensitive adhesivecompositions containing such block copolymer compositions.

BACKGROUND ART

Conventionally, various block copolymers includingpolystyrene/polyisoprene block copolymers have been used as basepolymers for pressure-sensitive adhesive compositions. Moreover, it isalso known that radial polystyrene/polyisoprene block copolymersrepresented by the following formula are used for such purposes.

(S−I)_(n)X

wherein S is a polystyrene block, I is a polyisoprene block, n is anatural number of 3 or greater, and X is a residue of a coupling agent.

Various coupling agents are used for the production of radial blockcopolymers. From the viewpoint of reactivity, bond stability, stabilizedsupply, cost and the like, tetrafunctional silicon compounds are amongthe most preferred coupling agents. Accordingly, several attempts havebeen made to synthesize a styrene/isoprene block copolymer forpressure-sensitive adhesive use by using a tetrafunctional siliconcompound as a coupling agent (see, for example, Japanese PatentLaid-Open No. 266156 /'89, Japanese Patent Laid-Open No. 138547/'95, andPublished Japanese Translation of PCT International Publication No.511260/'97). In these attempts, however, the resulting copolymerscarcely contains a four-branch polymer in spite of the use of atetrafunctional coupling agent, and its principal component is athree-branch polymer.

It is disclosed in Japanese Patent Laid-Open No. 337625/'96 that aradial polystyrene/polyisoprene block copolymer can be obtained byreacting polyisoprene with a silicon tetrachloride coupling agent, andthen adding thereto a cyclohexane solution of a separately formedpolystyrene/polyisoprene block copolymer and a coupling acceleratorcomprising 1,2-dimethoxyethane. However, this radialpolystyrene/polyisoprene block copolymer is an unsymmetrical blockcopolymer consisting of two polyisoprene arms and twopolystyrene/polyisoprene block copolymer arms.

It is disclosed in Published Japanese Translation of PCT InternationalPublication No. 511260/'97 that a block copolymer composition having afour-branch structure as a principal component can be obtained byintroducing a small amount of polybutadiene chains at the polymerizingend of the diblock polymer and then reacting the resulting copolymerwith a tetrafunctional coupling agent to form a four-branch polymer.However, its adhesive properties are markedly reduced by the influenceof the polybutadiene chains, and the resulting pressure-sensitiveadhesive composition fails to have satisfactory performance.

WO98/18840 discloses a poly(aromatic vinyl)/polyisoprene block copolymercomprising 5 to 50% by weight of a four-branch polymer and 50 to 95% byweight of a diblock polymer, and a pressure-sensitive adhesivecomposition containing this block copolymer. However, from the viewpointof the balance between tack at low temperatures and holding power, thereis yet room for improvement in this poly(aromatic vinyl)/polyisopreneblock copolymer.

An object of the present invention is provide a radial poly(aromaticvinyl)/polyisoprene block copolymer composition having a well-balancedcombination of tack at low temperatures and holding power and exhibitinghigh values therefor, a process for producing the same, and apressure-sensitive adhesive composition using the same.

DISCLOSURE OF THE INVENTION

The present inventors made intensive investigations with a view toaccomplishing the above object. As a result, it has now been found thata poly(aromatic vinyl)/polyisoprene block copolymer compositioncomprising (a) 1 to 34% by weight of a diblock polymer, (b) 34 to 99% byweight of a four-branch polymer, and (c) 0 to 50% by weight of at leastone branched polymer selected from the group consisting of a two-branchpolymer and a three-branch polymer can readily be obtained by preparinga poly(aromatic vinyl)/polyisoprene block copolymer comprising a diblockcopolymer having a poly(aromatic vinyl) block A and a polyisoprene blockB joined to each other, and then reacting it with a coupling agenthaving a functionality of 4 or higher, and that a pressure-sensitiveadhesive agent comprising this poly(aromatic vinyl)/polyisoprene blockcopolymer composition and a tackifier resin provides an excellentadhesive.

Thus, according to the present invention, there is provided apoly(aromatic vinyl)/polyisoprene block copolymer composition comprising1 to 34% by weight of a diblock polymer represented by the followingGeneral Formula 1 and referred to as component (a), 34 to 99% by weightof a four-branch polymer represented by the following General Formula 2and referred to as component (b), and 0 to 50% by weight of at least onebranched polymer selected from the group consisting of a two-branchpolymer represented by the following General Formula 3 and athree-branch polymer represented by the following General Formula 4, andreferred to as component (c), wherein the poly(aromaticvinyl)/polyisoprene block copolymer composed of component (a), component(b) and component (c) has a weight-average molecular weight (Mw) of260,000 to 500,000, and the poly(aromatic vinyl)/polyisoprene blockcopolymer has a poly(aromatic vinyl) block content of 5 to 24% byweight.

A¹-B¹  General Formula 1

wherein A¹ is a poly(aromatic vinyl) block having a weight-averagemolecular weight (Mw) of 9,000 to 20,000, and B¹ is a polyisopreneblock.

(A²-B²)₄X²  General Formula 2

wherein A² is a poly(aromatic vinyl) block having a weight-averagemolecular weight (Mw) of 9,000 to 20,000, B² is a polyisoprene block,and X² is a residue of a coupling agent having a functionality of 4 orhigher.

(A³-B³)₂X³  General Formula 3

wherein A³ is a poly(aromatic vinyl) block, B³ is a polyisoprene block,and X³ is a residue of a coupling agent having a functionality of 2 orhigher.

(A⁴-B⁴)₃X⁴  General Formula 4

wherein A⁴ is a poly(aromatic vinyl) block, B⁴ is a polyisoprene block,and X⁴ is a residue of a coupling agent having a functionality of 3 orhigher.

Moreover, according to the present invention, there is also provided aprocess for producing a poly(aromatic vinyl)/polyisoprene blockcopolymer composition, the process comprising the steps of (1) bringingan organolithium initiator into contact with an aromatic vinyl monomerto form a poly(aromatic vinyl) block A having an active polymerizingend, (2) adding isoprene so as to form an A-B block copolymer in which apolyisoprene block B having an active polymerizing end is directlyjoined to the poly(aromatic vinyl) block A, and (3) reacting the A-Bblock copolymer with a coupling agent having a functionality of 4 orhigher in the presence of a coupling accelerator so as to convert 34 to99% by weight of the A-B block copolymer into the four-branch polymerrepresented by General Formula 2.

Furthermore, according to the present invention, there is also provideda pressure-sensitive adhesive composition comprising the poly(aromaticvinyl)/polyisoprene block copolymer composition and a tackifier resin.

EMBODIMENTS OF THE INVENTION

[(a) Diblock Polymer]

The diblock polymer used as component (a) in the present invention is adiblock polymer represented by the following General Formula 1, and aresidue of a coupling agent may be added to an end of the polyisopreneblock B¹.

A¹-B¹  General Formula 1

wherein A¹ is a poly(aromatic vinyl) block and B¹ is a polyisopreneblock.

No particular limitation is placed on the type of the aromatic vinylmonomer used for the synthesis of the diblock polymer. Specific examplesthereof include styrene, α-methylstyrene, vinyltoluene andvinylnaphthalene, and styrene is preferred. These aromatic vinylmonomers may be used in admixture of two or more.

No particular limitation is placed on the proportion of thepoly(aromatic vinyl) block A¹ in the diblock polymer. Since thepoly(aromatic vinyl) block content of the block copolymer composed ofcomponents (a), (b) and (c) as will be described later is usually in therange of 5 to 24% by weight, preferably 10 to 18% by weight, and morepreferably 11 to 14% by weight, it is preferable to determine itsproportion so as to satisfy that condition. Usually, the proportion ofthe poly(aromatic vinyl) block A¹ in the diblock polymer is preferablyin the range of 5 to 24% by weight, more preferably 10 to 18% by weight,and most preferably 11 to 14% by weight.

No particular limitation is placed on the molecular weight of thediblock polymer. However, since the weight-average molecular weight (Mw)of the block copolymer composed of components (a), (b) and (c) in thepresent invention is limited as will be described later, it is necessaryto determine the molecular weight of the diblock polymer so as tosatisfy that condition. When the block copolymer composition of thepresent invention is produced by the production process of the presentinvention which will be described later, the weight-average molecularweight (Mw) of the diblock polymer as determined on a polystyrene basisby gel permeation chromatography (GPC) is usually in the range of 60,000to 250,000, preferably 70,000 to 230,000, and more preferably 80,000 to220,000.

The weight-average molecular weight (Mw) of the poly(aromatic vinyl)block A of the diblock polymer as determined on a polystyrene basis bygel permeation chromatography (GPC) is usually in the range of 9,000 to20,000, preferably 9,500 to 17,000, and more preferably 10,000 to15,000. If the weight-average molecular weight of the poly(aromaticvinyl) block A¹ is unduly low, the resulting pressure-sensitive adhesivecomposition will show a reduction in holding power. Conversely, if it isunduly high, the resulting pressure-sensitive adhesive composition willshow a reduction in tack.

No particular limitation is placed on the process for the preparation ofthe diblock polymer. Usually, an organolithium initiator is brought intocontact with an aromatic vinyl monomer to form a poly(aromatic vinyl)block A¹ having an active polymerizing end. Then, isoprene is added tothe reaction system so as to form a diblock polymer A¹-B¹ in which apolyisoprene block B¹ having an active polymerizing end is directlyjoined to the poly(aromatic vinyl) block A¹. The organolithium initiatorand other materials used for this purpose are the same as theorganolithium initiator and other materials used in the process forproducing the block copolymer of the present invention which will bedescribed later.

[(b) Four-branch Polymer]

The four-branch polymer constituting component (b) of the blockcopolymer composition of the present invention has a structurerepresented by the following General Formula 2.

(A²-B²)₄X²  General Formula 2

wherein A² is a poly(aromatic vinyl) block, B² is a polyisoprene block,and X² is a residue of a coupling agent having a functionality of 4 orhigher.

The aromatic vinyl monomer used for the four-branch polymer may be thesame as the aromatic vinyl monomer used for the preparation of thediblock polymer.

The weight-average molecular weight (Mw) of the poly(aromatic vinyl)block A² in the four-branch polymer is similar to the weight-averagemolecular weight (Mw) of the poly(aromatic vinyl) block A of the diblockpolymer.

The proportion of the poly(aromatic vinyl) block A² in the four-branchpolymer is similar to the proportion of the poly(aromatic vinyl) blockA¹ of the diblock polymer.

The four-branch polymer is formed by reacting a A²-B² block copolymerbeing a diblock polymer with a coupling agent having a functionality of4 or higher and preferably a tetrafunctional coupling agent.

[(c) Two-branch Polymer and Three-branch Polymer]

The block copolymer composition of the present invention may contain atleast one branched polymer selected from the group consisting of atwo-branch polymer represented by the following General Formula 3 and athree-branch polymer represented by the following General Formula 4.

(A³-B³)₂X³  General Formula 3

wherein A³ is a poly(aromatic vinyl) block, B³ is a polyisoprene block,and X³ is a residue of a coupling agent having a functionality of 2 orhigher.

(A⁴-B⁴)₃X⁴  General Formula 4

wherein A⁴ is a poly(aromatic vinyl) block, B⁴ is a polyisoprene block,and X⁴ is a residue of a coupling agent having a functionality of 3 orhigher.

The aromatic vinyl monomer used for the two-branch polymer and thethree-branch polymer may be the same as the aromatic vinyl monomer usedfor the diblock polymer.

The weight-average molecular weights (Mw) of the poly(aromatic vinyl)blocks A³ and A⁴ in the two-branch polymer and the three-branch polymerare both similar to the weight-average molecular weight of thepoly(aromatic vinyl) block A¹ of the diblock polymer.

The proportion of the poly(aromatic vinyl) block A³ in the two-branchpolymer and the proportion of the poly(aromatic vinyl) block A⁴ in thethree-branch polymer are both similar to the proportion of thepoly(aromatic vinyl) block A¹ of the diblock polymer.

[Block Copolymer Composition]

The poly(aromatic vinyl)/polyisoprene block copolymer composition of thepresent invention comprises the diblock polymer constituting component(a) in an amount of 1 to 34% by weight, preferably 1 to 30% by weight,and more preferably 1 to 20% by weight; the four-branch polymerconstituting component (b) in an amount of 34 to 99% by weight,preferably 45 to 99% by weight, and more preferably 55 to 99% by weight;and at least one branched polymer selected from the group consisting ofthe two-branch polymer and the three-branch polymer, and constitutingcomponent (c), in an amount of 0 to 50% by weight, preferably 0 to 40%by weight, and more preferably 0 to 30% by weight, provided that theamount of component (c) is less than the amount of component (b).Moreover, the amount of the two-branch polymer is preferably not greaterthan 10% by weight, more preferably not greater than 2% by weight, andmost preferably not greater than 1.5% by weight. Furthermore, the amountof component (b) based on the total amount of components other than thediblock polymer constituting component (a) in the block copolymercomposition is preferably not less than 50% by weight, more preferablynot less than 55% by weight, and most preferably not less than 60% byweight.

If the amount of component (a) in the block copolymer composition isunduly large, the holding power will be reduced. If the amount ofcomponent (b) in the block copolymer composition is unduly small, theholding power will be reduced. If the amount of component (c) in theblock copolymer composition is unduly large, the effects of the presentinvention will be lessened. Especially if the amount of the two-branchpolymer is unduly large, the effects of the present invention will belessened.

The weight-average molecular weight (Mw) of the block copolymer composedof components (a), (b) and (c), as determined on a polystyrene basis bygel permeation chromatography (GPC), is in the range of 260,000 to500,000, preferably 260,000 to 470,000, more preferably 280,000 to450,000, and most preferably 300,000 to 450,000. If this weight-averagemolecular weight is unduly low, the holding power will be reduced. If itis unduly high, the viscosity will be increased to cause a reduction inprocessability.

No particular limitation is placed on the molecular weight distributionof the block copolymer composed of components (a), (b) and (c). However,the ratio (Mw/Mn) of the weight-average molecular weight (Mw), asdetermined on a polystyrene basis by gel permeation chromatography(GPC), to the number-average molecular weight (Mn) is preferably notgreater than 4, more preferably not greater than 3, and most preferablynot greater than 2. In such a case, the pressure-sensitive adhesivecomposition formed therefrom preferably has a highly balancedcombination of holding power and tack.

The poly(aromatic vinyl) block content of the poly(aromaticvinyl)/polyisoprene block copolymer composed of components (a), (b) and(c) is in the range of 5 to 24% by weight, preferably 10 to 18% byweight, and more preferably 11 to 14% by weight. If the poly(aromaticvinyl) block content of the poly(aromatic vinyl)/polyisoprene blockcopolymer composed of components (a), (b) and (c) is unduly low, theresulting pressure-sensitive adhesive composition will have insufficientholding power. If it is unduly high, the resulting pressure-sensitiveadhesive composition will have insufficient tack.

[Process for Producing the Block Copolymer Composition]

No particular limitation is placed on the process for producing thepoly(aromatic vinyl)/polyisoprene block copolymer composition of thepresent invention. For example, the block copolymer composition of thepresent invention may be prepared by separately synthesizing (a) adiblock polymer, (b) a four-branch polymer, and optionally (c) at leastone branched polymer selected from a two-branch polymer and athree-branch polymer, purifying these polymers, and mixing thesepolymers in predetermined proportions. Alternatively, the blockcopolymer composition of the present invention may also be producedaccording to the production process of the present invention whichcomprises the steps of (1) bringing an organohthium initiator intocontact with an aromatic vinyl monomer to form a poly(aromatic vinyl)block A having an active polymerizing end, (2) adding isoprene so as toform an A-B block copolymer in which a polyisoprene block B having anactive polymerizing end is directly joined to the poly(aromatic vinyl)block A, and (3) reacting the A-B block copolymer with a coupling agenthaving a functionality of 4 or higher in the presence of a couplingaccelerator so as to convert 34 to 99% by weight of the A-B blockcopolymer into the four-branched polymer represented by General Formula2.

The poly(aromatic vinyl)/polyisoprene block copolymer compositionproduced by the production process of the present invention is apoly(aromatic vinyl)/polyisoprene block copolymer composition comprising1 to 34% by weight of a diblock polymer represented by the followingGeneral Formula 5 and referred to as component (X), 34 to 99% by weightof a four-branched polymer represented by the following General Formula6 and referred to as component (Y), and 0 to 50% by weight of at leastone branched polymer selected from the group consisting of atwo-branched polymer represented by the following General Formula 7 anda three-branched polymer represented by the following General Formula 8,and referred to as component (Z).

A⁵-B⁵  General Formula 5

wherein A⁵ is a poly(aromatic vinyl) block, and B⁵ is a polyisopreneblock.

(A⁶-B⁶)₄X⁶  General Formula 6

wherein A⁶ is a poly(aromatic vinyl) block, B⁶ is a polyisoprene block,and X⁶ is a residue of a coupling agent having a functionality of 4 orhigher.

(A⁷-B⁷)₂X⁷  General Formula 7

wherein A⁷ is a poly(aromatic vinyl) block, B⁷ is a polyisoprene block,and X⁷ is a residue of a coupling agent having a functionality of 2 orhigher.

 (A⁸-B⁸)₃X⁸  General Formula 8

wherein A⁸ is a poly(aromatic vinyl) block, B⁸ is a polyisoprene block,and X8 is a residue of a coupling agent having a functionality of 3 orhigher.

The production process of the present invention makes it possible toproduce poly(aromatic vinyl)/polyisoprene block copolymer compositionscontaining a high proportion of the four-branch polymer represented byGeneral Formula 6.

Step (1) in the production process of the present invention is a stepfor forming a poly(aromatic vinyl)/polyisoprene block copolymercomposition. This step is carried out by bringing an organolithiuminitiator into contact with an aromatic vinyl monomer in apolymerization solvent so as to polymerize the aromatic vinyl monomer,and the poly(aromatic vinyl) block A so formed has an activepolymerizing end.

As the organolithium initiator, there may be used any well-knownorganolithium compound that can initiate the polymerization of aromaticvinyl monomers and isoprene. Specific examples thereof includeorgano-monolithium initiators such as methyllithium, n-propyllithium,n-butyllithium and sec-butyllithium. Among them, n-butyllithium ispreferred. The amount of organolithium initiator used can be determinedby calculation according to the desired molecular weight of the polymer,in a manner well known to those skilled in the art.

The aromatic vinyl monomer used is the same as the aromatic vinylmonomer used for the preparation of the diblock polymer.

No particular limitation is placed on the type of the polymerizationsolvent, provided that it is inert to the organolithium initiator. Forexample, there is used an open-chain hydrocarbon solvent, a cyclichydrocarbon solvent or a mixture thereof. Examples of the open-chainhydrocarbon solvent include open-chain alkanes and alkenes of 4 to 6carbon atoms, such as n-butane, isobutane, n-hexane and mixturesthereof; 1-butene, isobutylene, trans-2-butene, cis-2-butene andmixtures thereof; n-pentane, trans-2-pentane, neo-pentane and mixturesthereof. Specific examples of the cyclic hydrocarbon solvent includearomatic hydrocarbons such as benzene, toluene and xylene; and alicyclichydrocarbons such as cyclohexane. From the viewpoint of controllabilityof the polymerization temperature and controllability of the molecularweight distributions of the polymer block formed from the aromatic vinylmonomer and of the poly(aromatic vinyl)/polyisoprene block copolymercomposed of components (X), (Y) and (Z), an open-chain hydrocarbonsolvent of 4 to 6 carbon atoms and a cyclic hydrocarbon solvent arepreferably used by mixing them in a weight ratio of 5:95 to 50:50 andmore preferably 10:90 to 40:60.

In the production process of the present invention, the polymerizationmay be carried out in the presence of a polar compound so as to controlthe polymerization rate of the aromatic vinyl monomer and the molecularweight distribution of the poly(aromatic vinyl) block. As the polarcompound, an aromatic ether, aliphatic ether or tertiary amine having adielectric constant of 2.5 to 5.0 as measured at 25° C. may preferablybe used. Specific examples of such polar compounds include aromaticethers such as diphenyl ether and anisole; aliphatic ethers such asdiethyl ether and dibutyl ether; tertiary monoamine such astrimethylamine, triethylamine and tripropylamine; and tertiarypolyamines such as N,N,N′,N′-tetra-methylethylenediamine andN,N,N′,N′-tetraethylethylene-diamine.

These polar compounds may be used in admixture of two or more. Theamount of polar compound used may vary according to the type thereof.For example, aliphatic ethers are preferably used in an amount of 0.001to 50 moles and more preferably 0.005 to 10 moles, per mole of theorganolithium initiator. Tertiary polyamines are preferably used in anamount of 0.001 to 0.1 mole, more preferably 0.005 to 0.08 mole, andmost preferably 0.01 to 0.06 mole, per mole of the organolithiuminitiator. If the amount of polar compound used is unduly small, it willbe difficult to accomplish the purpose of controlling the reaction rateand the molecular weight distribution. Even if it is added in excess,the effect obtained thereby will not be in proportion to the amountadded. Moreover, if an excessive amount of a tertiary polyamine is addedin step (1), it will exist in excess in step (2). This may detract fromthe low-temperature characteristics of the resulting pressure-sensitiveadhesive composition.

These polar compounds are also suitable for use as coupling acceleratorsin step (3) of the production process of the present invention.

Then, in step (2), isoprene is added to the polymerization system so asto form an A-B block copolymer in which a polyisoprene block B having anactive polymerizing end is directly joined to the poly(aromatic vinyl)block A. In order to control the heat of reaction, it is preferable toadd isoprene continuously, instead of adding it at a time.

Furthermore, in step (3), a coupling agent having a functionality of 4or higher, preferably a tetrafunctional coupling agent, is added inorder to convert 34 to 99% by weight, preferably 45 to 99% by weight,and more preferably 55 to 99% by weight of the A-B block copolymer intothe four-branch polymer represented by General Formula 6. Inconsequence, the diblock polymer represented by General Formula 5 andconstituting component (X), and the at least one branched polymerselected from the group consisting of the two-branch polymer representedby General Formula 7 and the three-branch polymer represented by GeneralFormula 8, and constituting component (Z), may be contained in thecomposition. In this case, A⁵, A⁶, A⁷ and A⁸ are the same as A, and B⁵,B⁶, B⁷ and B⁸ are the same as B. Moreover, the diblock polymer maycontain the A-B block copolymer having a residue of the coupling agentjoined to the B end. Also in this case, the diblock polymer is dealtwith as such.

The method for converting 34 to 99% by weight of the A-B block copolymerinto the four-branch polymer is carried out by reacting the A-B blockcopolymer with the coupling agent having a functionality of 4 or higherin the presence of a coupling accelerator.

Specific examples of the coupling accelerator are the same as thecompounds described above as specific examples of the polar compound.

The amount of coupling accelerator used in step (1) through step (3)(including the amount used as a polar compound) may vary according tothe type thereof, the type and concentration of the coupling agent, andthe like. For example, aliphatic ethers are preferably used in an amountof 0.001 to 50 moles and more preferably 0.005 to 10 moles, per mole ofthe organolithium initiator. Tertiary polyamines are used in an amountof 0.02 to 1 mole, preferably 0.05 to 0.5 mole, and more preferably 0.08to 0.3 moles, per mole of the organolithium initiator. Thus, apredetermined proportion of the A-B block copolymer is converted intothe four-branch polymer by controlling the reaction time or otherparameters in the presence of the coupling accelerator.

No particular limitation is placed on the time at which the couplingaccelerator is added to the reaction system. However, if a tertiarypolyamine functioning as a polar compound in the aromatic vinylpolymerization step (1) exists in excess in the isoprene polymerizationstep (2), it will detract from the low-temperature characteristics ofthe resulting pressure-sensitive adhesive composition as describedpreviously. Accordingly, where a tertiary polyamine is used both as apolar compound and as a coupling accelerator, it is important to controlits time of addition and its amount used. For example, in the aromaticvinyl polymerization step (1), a tertiary polyamine may be added to thereaction system in such an amount as to function as a polar compound inthe aromatic vinyl polymerization step (1), exert little influence onthe isoprene polymerization step (2), and function as a couplingaccelerator in the coupling step (3). Alternatively, in the aromaticvinyl polymerization step (1), a tertiary polyamine may be added to thereaction system in such an amount as to function as a polar compound inthe aromatic vinyl polymerization step (1) and exert little influence onthe isoprene polymerization step (2). Then, after the isoprenepolymerization step (2), the tertiary polyamine may further be added soas to function as a coupling accelerator in the coupling step (3).

The amount of tertiary polyamine further added after step (2) is usuallyin the range of 0 to 0.4 mole, preferably 0.01 to 0.4 mole, and morepreferably 0.01 to 0.2 mole, per mole of the organolithium initiator.

When a polar compound is added in step (1) and a coupling accelerator isfurther added in step (3), the coupling efficiency can be enhanced toincrease the content of the four-branch polymer.

As the coupling agent having a functionality of 4 or higher, ahalogenated silane is preferably used. Specific examples thereof includetetrachlorosilane and tetrabromosilane. In the present invention, thesecoupling agents may be used in admixture of two or more, so long as thepurposes of the present invention are not interfered with. Similarly,they may be used in combination with bifunctional or trifunctionalcoupling agents.

In the present invention, the total amount of coupling agent used issuitably chosen so that the aforesaid components (X), (Y) and (Z) willbe present in predetermined proportions. The amount of coupling agent isusually in the range of 0.05 to 0.4 mole and preferably 0.085 to 0.3mole, per mole of the organolithium initiator.

The appropriate amount of coupling agent used can be determined bycalculation according to the proportions of the four-branch polymercomponent and the diblock polymer component in the desired blockcopolymer. However, since there are problems, for example, with aninactivation of the organolithium initiator or the coupling agent, it iscommon practice to determine its optimum amount by preliminaryexperiments. In order to control the degree of coupling, a reactionstopper such as methanol may also be used as required.

In step (3), the coupling agent may be added at a time or in two or moredivided portions. Preferably, the coupling agent is added in two dividedportions. When the coupling agent is added in two divided portions, thefirst portion usually comprises 40 to 90 mole %, preferably 50 to 90mole %, of the total amount of coupling agent used and the secondportion usually comprises 10 to 60 mole %, preferably 10 to 50 mole %,of the total amount of coupling agent used. The interval of time betweenthe two additions of the coupling agent is usually not less than 5minutes and preferably not less than 10 minutes. The total time of thecoupling reaction is usually in the range of 1 to 5 hours. By adding thecoupling agents in divided portions, the time required for the couplingreaction can be markedly reduced and, moreover, a high-molecular-weightpoly(aromatic vinyl)/polyisoprene block copolymer composition containinga higher proportion of a four-branch polymer can be obtained.

As soon as the coupling reaction has proceeded to an appropriate extent,the reaction is stopped. This can be accomplished, for example, byadding a reaction stopper (e.g., water, methanol or an acid) toinactivate the active polymerizing species. Subsequently, the polymer isseparated, for example, by a well-known polymer separation techniquesuch as steam stripping, and then dried to obtain the desiredpoly(aromatic vinyl)/polyisoprene block copolymer composition comprisingthe diblock polymer and the four-branch polymer.

No particular limitation is placed on the weight-average molecularweight (Mw) of the poly(aromatic vinyl)/polyisoprene block copolymercomposed of components (X), (Y) and (Z) and contained in thepoly(aromatic vinyl)/polyisoprene block copolymer composition producedby the production process of the present invention. However, it isusually in the range of 260,000 to 500,000.

No particular limitation is placed on the poly(aromatic vinyl) blockcontent of the poly(aromatic vinyl)/polyisoprene block copolymercomposed of components (X), (Y) and (Z) and contained in thepoly(aromatic vinyl)/polyisoprene block copolymer composition producedby the production process of the present invention. However, it isusually in the range of 5 to 24% by weight.

No particular limitation is placed on the weight-average molecularweights of the poly(aromatic vinyl) blocks of components (X), (Y) and(Z) contained in the poly(aromatic vinyl)/polyisoprene block copolymercomposition produced by the production process of the present invention.However, they are usually in the range of 9,000 to 20,000.

According to the production process of the present invention, thepoly(aromatic vinyl)/polyisoprene block copolymer composition comprisingthe aforesaid components (a), (b) and (c) can preferably be produced.When the poly(aromatic vinyl)/polyisoprene block copolymer compositioncomprising the aforesaid components (a), (b) and (c) is producedaccording to the production process of the present invention, components(X), (Y) and (Z) described in connection with the production process ofthe present invention correspond to components (a), (b) and (c),respectively.

[Pressure-sensitive Adhesive Composition]

The pressure-sensitive adhesive composition of the present inventioncomprises the block copolymer composition of the present invention and atackifier resin.

No particular limitation is placed on the type of the tackifier resin,and there may used, for example, well-known natural resin type tackifierresins and synthetic resin type tackifier resins which are used incommon pressure-sensitive adhesive compositions. The natural resin typetackifier resins include rosin type resins and terpene type resins.Usable rosin type resins include, for example, rosins such as gum rosin,tall rosin and wood rosin; modified rosins such as hydrogenated rosin,disproportionated rosin and polymerized rosin; and rosin esters such asglycerol esters and pentaerythritol esters of modified rosins. Usableterpene type resins include, for example, terpene resins such asα-pinene resin, β-pinene resin and dipentene (limonene) resin, as wellas aromatic modified terpene resins, hydrogenated terpene resins andterpene phenol resin. The synthetic resin type tackifier resins arebroadly classified into polymerization type tackifier resins andcondensation type tackifier resins. Usable polymerization type tackifierresins include petroleum resins such as aliphatic (C5 type) petroleumresins, aromatic (C9 type) petroleum resins, copolymerized (C5-C9 type)petroleum resins, hydrogenated petroleum resins, alicyclic petroleumresins (e.g., dicyclopentadiene resin and other alicyclic petroleumresins); coumarone-indene resin; and pure monomer type petroleum resinssuch as styrene resin and substituted styrene resins. Usablecondensation type tackifier resins phenolic resins such as alkylphenolresins and rosin-modified phenol resins; xylene resins; and the like.Among them, petroleum resins are preferred. Especially preferred arealipatic (C5 type) petroleum resins and copolymerized (C5-C9 type)petroleum resins, particularly having a softening point of 70 to 120° C.Among the copolymerized (C5-C9 type) petroleum resins, those containing5 to 15% by weight of an aromatic monomer having 8 or 9 carbon atoms areespecially preferred.

The amount of tackifier resin used is in the range of 10 to 500 parts byweight, preferably 50 to 350 parts by weight, and more preferably 70 to250 parts by weight, per 100 parts by weight of the poly(aromaticvinyl)/polyisoprene block copolymer composition.

If necessary, the pressure-sensitive adhesive composition of the presentinvention may further contain compounding ingredients such as rubbercomponents, softening agents, antioxidants, heat stabilizers,ultraviolet absorbers and fillers.

Usable rubber components include styrene-based block copolymers such aspolystyrene-polybutadiene-polystyrene block copolymer,polystyrene-polyethylene-polybutylene-polystyrene block copolymer andpolystyrene-polyethylene-polypropylene-polystyrene block copolymer;diene rubbers such as butadiene rubber and isoprene rubber; naturalrubber; and the like. These rubber components may be used in admixtureof two or more. The amount of rubber component used is not particularlylimited, but may be determined according to the characteristics of eachrubber component and the characteristics desired for thepressure-sensitive adhesive composition.

Usable softening agents include extender oils which have conventionallybe used in pressure-sensitive adhesive compositions, such as aromaticprocess oils, paraffinic process oils and naphthenic process oils;liquid polymers such as polybutene and polyisobutylene; and the like.Among them, extender oils such as paraffinic process oils and naphthenicprocess oils are preferred. These softening agents may be used inadmixture of two or more. The amount of softening agent used ispreferably in the range of not greater than 500 parts by weight, morepreferably not greater than 300 parts by weight, and most preferably notgreater than 150 parts by weight, per 100 parts by weight of thepoly(aromatic vinyl)/polyisoprene block copolymer composition. If theamount of softening agent used is unduly large, bleeding may occur.

Usable antioxidants include hindered phenol compounds such as2,6-di-tert-butyl-p-cresol, pentaerythrityltetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butylanilino)-1,3,5-triazine;thiodicarboxylate esters such as dilauryl thiodipropionate; phosphorousesters such as tris(nonylphenyl) phosphite and4,4-butylidene-bis(3-methyl-6-butylphenyl) ditridecyl phosphite; and thelike. These antioxidants may be used in admixture of two or more. Theamount of antioxidant used may be determined according to thecharacteristics of each antioxidant and the characteristics desired forthe pressure-sensitive adhesive composition.

Industral Applicability

The pressure-sensitive adhesive composition of the present invention maybe used as a solution type pressure-sensitive adhesive by dissolving itin n-hexane, cyclohexane, benzene, toluene or the like; as an emulsiontype pressure-sensitive adhesive by dispersing it in water with the aidof an emulsifier; or as a solvent-less hot-melt type pressure-sensitiveadhesive or bonding adhesive. It is especially suitable for use as ahot-melt type pressure-sensitive adhesive.

The hot-melt type pressure-sensitive adhesive is preferably used for theproduction of various pressure-sensitive adhesive tapes, labels,dedusting rollers and the like. For example, such pressure-sensitiveadhesive tapes can be used in a wide variety of application fieldsincluding packaging, office work, double-coated tapes, masking,electrical insulation and the like, and exhibit high holding power andexcellent tack at low temperatures.

Best Mode for Carrying Out the Invention

The present invention is more specifically explained with reference tothe following examples.

The weight-average molecular weight of a polystyrene block is aweight-average molecular weight (Mw) on a polystyrene basis, which wasdetermined by sampling polystyrene at the time of addition of isopreneduring the production of a block copolymer and subjecting this sample togel permeation chromatography (GPC) using tetrahydrofuran as a carrier.The weight-average molecular weight of a block copolymer composed ofcomponents (a), (b) and (c) is also a weight-average molecular weight(Mw) as determined on a polystyrene basis by gel permeationchromatography (GPC) using tetrahydrofuran as a carrier. The makeup of acopolymer composition was determined from the peak areas of variouscomponents which were recorded by the same gel permeation chromatography(GPC).

Holding power was evaluated according to PSTC-7 (a holding power testprescribed by the American Pressure Sensitive Tape Council).Specifically, a piece of pressure-sensitive adhesive tape having a widthof 10 mm was adhered to paper (K-liner) so as to give a 10 mm×10 mmbonded area, and its holding power was measured at 23° C.

A paper peeling test for evaluating tack at low temperatures was carriedout as follows. A pressure-sensitive adhesive tape to be tested was cutinto test pieces measuring 25 mm×70 mm. At least 3 hours before thestart of the test, these test pieces, together with a corrugated boardas an adherend and a 500 g metal roller, was inserted into athermostatic chamber at 5° C. The corrugated board was placed in thethermostatic chamber at 5° C., and a test piece of thepressure-sensitive adhesive tape was affixed thereto under constantconditions without applying any force thereon. The test piece was bondedunder pressure by moving the metal roller thereon in a singlereciprocating motion, and then peeled quickly at an angle of 90 degrees.The amount of paper fiber adhering to the peeled test piece of thepressure-sensitive adhesive tape was scored by comparison with standardsamples [5 ranks ranging from 5 (fiber adhering to the whole surface ofthe test piece) to 1 (no fiber adhering to the test piece)].Measurements were made with 6 test pieces, and the average of themeasured values was calculated.

EXAMPLE 1

[Preparation of a Block Copolymer Composition]

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,4.0 millimoles of N,N,N′,N′-tetramethylethylenediamine as a polarcompound and a coupling accelerator, and 80.0 millimoles ofn-butyllithium. After the reactor was heated to 60° C., 0.96 kg ofstyrene was added, and the resulting mixture was polymerized for 0.5hour. Subsequently, while 7.04 kg of isoprene was being continuouslyadded and the reaction temperature was being controlled so as to liebetween 60° C. and 70° C., the reaction mixture was further polymerizedfor 1.5 hours. Then, 19.0 millimoles of tetrachlorosilane as a couplingagent was added, and a coupling reaction was carried out for 1 hour.Thereafter, 50 ml of methanol as a reaction stopper and 40 g of2,6-di-tert-butyl-p-cresol as an antioxidant were added to the reactionmixture, followed by thorough mixing. The resulting mixed solution wasadded, little by little, to warm water heated to 85-95° C., so that thesolvent was evaporated. The polymer so recovered was pulverized anddried in vacuo at 60° C. to obtain a block copolymer composition A inaccordance with the present invention.

The styrene content of this block copolymer composition was 12% byweight, the weight-average molecular weight of the polystyrene block was12,300, and the weight-average overall molecular weight (Mw) asdetermined on a polystyrene basis was 343,000. This block copolymercomposition was composed of 43.7% by weight of a four-branch polymer,37.0% by weight of a three-branch polymer, 1.7% by weight of atwo-branch polymer, and 17.6% by weight of a diblock polymer.

[Pressure-sensitive Adhesive Composition]

100 parts by weight of the aforesaid block copolymer composition wasplaced in a kneader equipped with agitating blades. Then, 77.8 parts byweight of an aliphatic (C5 type) petroleum resin (with a softening pointof 96° C.; Quinton R100, manufactured by Nippon Zeon Co., Ltd.), 7.4parts by weight of a naphthenic process oil (Shell Flex 371,manufactured by Shell Chemical Co., Ltd.), and 0.92 part by weight of anantioxidant (Irganox 1010, manufactured by Ciba-Geigy) were addedthereto. After the atmosphere of the system was replaced by nitrogengas, the mixture was kneaded at 160-180° C. to prepare apressure-sensitive adhesive composition. A polyester film having athickness of 30 μm was coated with the pressure-sensitive adhesivecomposition to a thickness of 30 μm, and used for the evaluation of itsadhesive properties. The results thus obtained are shown in Table 1.

EXAMPLE 2

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,4.1 millimoles of tetramethylethylenediamine as a polar compound, and82.7 millimoles of n-butyllithium. After the reactor was heated to 60°C., 0.96 kg of styrene was added, and the resulting mixture waspolymerized for 0.5 hour. Subsequently, while 7.04 kg of isoprene wasbeing continuously added and the reaction temperature was beingcontrolled so as to lie between 60° C. and 70° C., the reaction mixturewas further polymerized for 1.5 hours. Then, 4.1 millimoles ofN,N,N′,N′-tetramethylethylenediamine and 17.8 millimoles oftetrachlorosilane as a coupling agent were added, and a couplingreaction was carried out for 1 hour. Thereafter, according to the sameprocedure as in Example 1, a reaction stopper and an antioxidant wereadded to the reaction mixture, the solvent was evaporated, and theresulting polymer was pulverized and dried to obtain a block copolymercomposition B. Properties of this block copolymer composition wereevaluated. Moreover, using this block copolymer composition, apressure-sensitive adhesive composition was prepared in the same manneras in Example 1, and its adhesive properties were evaluated. The resultsthus obtained are shown in Table 1.

EXAMPLE 3

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,3.6 millimoles of tetramethylethylenediamine as a polar compound, and71.6 millimoles of n-butyllithium. After the reactor was heated to 60°C., 0.88 kg of styrene was added, and the resulting mixture waspolymerized for 0.5 hour. Subsequently, while 7.12 kg of isoprene wasbeing continuously added and the reaction temperature was beingcontrolled so as to lie between 60° C. and 70° C., the reaction mixturewas further polymerized for 1.5 hours. Then, 3.6 millimoles oftetramethylethylenediamine as a coupling accelerator and 17.8 millimolesof tetrachlorosilane as a coupling agent were added, and a couplingreaction was carried out for 3 hours. Thereafter, according to the sameprocedure as in Example 1, a reaction stopper and an antioxidant wereadded to the reaction mixture, the solvent was evaporated, and theresulting polymer was pulverized and dried to obtain a block copolymercomposition C. Properties of this block copolymer composition wereevaluated.

100 parts by weight of this block copolymer composition was placed in akneader equipped with agitating blades. Then, 85.2 parts by weight of acopolymerized (C5-C9 type) petroleum resin (with a softening point of80° C.; Quinton N180, manufactured by Nippon Zeon Co., Ltd.) and 0.92part by weight of an antioxidant (Irganox 1010, manufactured byCiba-Geigy) were added thereto. After the atmosphere of the system wasreplaced by nitrogen gas, the mixture was kneaded at 160-180° C. toprepare a pressure-sensitive adhesive composition. Adhesion propertiesof this pressure-sensitive adhesive composition were evaluated in thesame manner as in Example 1. The results thus obtained are shown inTable 1.

EXAMPLE 4

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,4.2 millimoles of tetramethylethylenediamine as a polar compound, and84.6 millimoles of n-butyl rthium. After the reactor was heated to 60°C., 1.04 kg of styrene was added, and the resulting mixture waspolymerized for 0.5 hour. Subsequently, while 6.96 kg of isoprene wasbeing continuously added and the reaction temperature was beingcontrolled so as to lie between 60° C. and 70° C., the reaction mixturewas further polymerized for 1.5 hours. Then, 4.2 millimoles oftetramethylethylenediamine as a coupling accelerator and 16.8 millimolesof tetrachlorosilane as a coupling agent were added, and a couplingreaction was carried out for 1 hour. Thereafter, according to the sameprocedure as in Example 1, a reaction stopper and an antioxidant wereadded to the reaction mixture, the solvent was evaporated, and theresulting polymer was pulverized and dried to obtain a block copolymercomposition D. Properties of this block copolymer composition wereevaluated.

100 parts by weight of this block copolymer composition was placed in akneader equipped with agitating blades. Then, 85.2 parts by weight of acopolymerized (C5-C9 type) petroleum resin (with a softening point of80° C.; Quinton N180, manufactured by Nippon Zeon Co., Ltd.) and 0.92part by weight of an antioxidant (Irganox 1010, manufactured byCiba-Geigy) were added thereto. After the atmosphere of the system wasreplaced by nitrogen gas, the mixture was kneaded at 160-180° C. toprepare a pressure-sensitive adhesive composition. Adhesion propertiesof this pressure-sensitive adhesive composition were evaluated in thesame manner as in Example 1. The results thus obtained are shown inTable 1.

EXAMPLE 5

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,4.7 millimoles of tetramethylethylenediamine as a polar compound, and94.9 millimoles of n-butyllithium. After the reactor was heated to 60°C., 1.12 kg of styrene was added, and the resulting mixture waspolymerized for 0.5 hour. Subsequently, while 6.88 kg of isoprene wasbeing continuously added and the reaction temperature was beingcontrolled so as to lie between 60° C. and 70° C., the reaction mixturewas further polymerized for 1.5 hours. Then, 4.7 millimoles oftetramethylethylenediamine and 16.6 millimoles of tetrachlorosilane as acoupling agent were added, and a coupling reaction was carried out for 1hour. Thereafter, according to the same procedure as in Example 1, areaction stopper and an antioxidant were added to the reaction mixture,the solvent was evaporated, and the resulting polymer was pulverized anddried to obtain a block copolymer composition E. Properties of thisblock copolymer composition were evaluated.

100 parts by weight of this block copolymer composition was placed in akneader equipped with agitating blades. Then, 85.2 parts by weight of analiphatic (C5 type) petroleum resin (with a softening point of 70° C.;Quinton B170, manufactured by Nippon Zeon Co., Ltd.) and 0.92 part byweight of an antioxidant (Irganox 1010, manufactured by Ciba-Geigy) wereadded thereto. After the atmosphere of the system was replaced bynitrogen gas, the mixture was kneaded at 160-180° C. to prepare apressure-sensitive adhesive composition. Adhesion properties of thispressure-sensitive adhesive composition were evaluated in the samemanner as in Example 1. The results thus obtained are shown in Table 1.

EXAMPLE 6

A 30-liter pressure reactor was charged with 18.75 kg of cyclohexane,1.3 millimoles of N,N,N′,N′-tetramethylethylenediamine as a polarcompound, and 87.3 millimoles of n-butyl lithium. After the reactor washeated to 60° C., 0.96 kg of styrene was added, and the resultingmixture was polymerized for 0.5 hour. Subsequently, while 7.04 kg ofisoprene was being continuously added and the reaction temperature wasbeing controlled so as to lie between 60° C. and 70° C., the reactionmixture was further polymerized for 1.5 hours. Then, 8.7 millimoles ofN,N,N′,N′-tetramethylethylenediamine as a coupling accelerator, and 17.5millimoles of tetrachlorosilane as a coupling agent were added, and acoupling reaction was carried out for 15 minutes. Then, 4.4 millimolesof tetrachlorosilane was added and the coupling reaction was continuedfor an additional 45 minutes. Thereafter, a block copolymer compositionF was obtained in the same manner as in Example 1. Moreover, apressure-sensitive adhesive composition was prepared in the same manneras in Example 1, and its adhesive properties were evaluated. The resultsthus obtained are shown in Table 1.

EXAMPLE 7

A block copolymer G was obtained by carrying out reaction in the samemanner as in Example 6, except that 85.7 millimoles of n-butyllithiumwas used, 8.6 millimoles of N,N,N′,N′-tetramethylethy lenediamine as acoupling accelerator and 17.1 millimoles of tetrachlorosilane as acoupling agent were added after the polymerization of isoprene, and 8.6millimoles of tetrachlorosilane was further added after 15 minutes.Moreover, a pressure-sensitive adhesive composition was prepared in thesame manner as in Example 1, and its adhesive properties were evaluated.The results thus obtained are shown in Table 1.

EXAMPLE 8

A block copolymer H was obtained by carrying out reaction in the samemanner as in Example 6, except that 84.9 millimoles of n-butyllithiumwas used, 8.5 millimoles of N,N,N′,N′-tetramethylethylenediamine as acoupling accelerator and 12.7 millimoles of tetrachlorosilane as acoupling agent were added after the polymerization of isoprene, and 5.9millimoles of tetrachlorosilane was further added after 15 minutes.Moreover, a pressure-sensitive adhesive composition was prepared in thesame manner as in Example 1, and its adhesive properties were evaluated.The results thus obtained are shown in Table 1.

EXAMPLE 9

A block copolymer I was obtained by carrying out reaction in the samemanner as in Example 6, except that 1.8 millimoles ofN,N,N′,N′-tetramethylethylenediamine as a polar compound, 121.6millimoles of n-butyllithium, 1.52 kg of styrene, and 6.48 kg ofisoprene were used, 12.2 millimoles ofN,N,N′,N′-tetramethylethylenediamine as a coupling accelerator and 24.3millimoles of tetrachlorosilane as a coupling agent were added, and 6.1millimoles of tetrachlorosilane was further added after 15 minutes.

EXAMPLES 10 and 11

Pressure-sensitive adhesive compositions as shown in Table 1 wereprepared by using the block copolymer C obtained in Example 3, and theiradhesive properties were evaluated. The results thus obtained are shownin Table 1.

Comparative Example 1

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,1.2 millimoles of N,N,N′,N′-tetramethylethylenediamine as a polarcompound, and 81.3 millimoles of n-butyllithium. After the reactor washeated to 60° C., 0.96 kg of styrene was added, and the resultingmixture was polymerized for 0.5 hour. Subsequently, while 7.04 kg ofisoprene was being continuously added and the reaction temperature wasbeing controlled so as to lie between 60° C. and 70° C., the reactionmixture was further polymerized for 1.5 hours. Then, 18.8 millimoles oftetrachlorosilane was added, and a coupling reaction was carried out for1 hour. Thereafter, according to the same procedure as in Example 1, areaction stopper and an antioxidant were added to the reaction mixture,the solvent was evaporated, and the resulting polymer was pulverized anddried to obtain a block copolymer composition J. Properties of thisblock copolymer composition were evaluated. Moreover, using this blockcopolymer composition, a pressure-sensitive adhesive composition wasprepared in the same manner as in Example 1, and its adhesive propertieswere evaluated. The results thus obtained are shown in Table 1.

Comparative Examples 2 to 4

Using three commercially available polystyrene-polyisoprene-polystyreneblock copolymer compositions K, L and M, pressure-sensitive adhesivecompositions were prepared according to the formulations shown in Table1, and their characteristics and adhesive properties were evaluated inthe same manner as before. The results thus obtained are shown in Table1.

Comparative Example 5

A 50-liter pressure reactor was charged with 18.75 kg of cyclohexane,and 95.3 millimoles of sec-butyllithium. After the reactor was heated to60° C., 1.68 kg of styrene was added, and the resulting mixture waspolymerized for 0.5 hour. Subsequently, while 6.32 kg of isoprene wasbeing continuously added and the reaction temperature was beingcontrolled so as to lie between 60° C. and 70° C., the reaction mixturewas further polymerized for 1.5 hours. Then, 0.02 kg of butadiene wasadded and polymerized. Moreover, 23.9 millimoles of tetrachlorosilanewas added, and a coupling reaction was carried out for 1 hour.Thereafter, according to the same procedure as in Example 1, a reactionstopper and an antioxidant were added to the reaction mixture, thesolvent was evaporated, and the resulting polymer was pulverized anddried to obtain a block copolymer composition N. Properties of thisblock copolymer composition were evaluated. Moreover, using this blockcopolymer composition, a pressure-sensitive adhesive composition wasprepared in the same manner as in Example 1, and its adhesive propertieswere evaluated. The results thus obtained are shown in Table 1.

TABLE 1 Example 1 2 3 4 5 6 7 8 Block copolymer composition A B C D E FG H Styrene content (wt. %) 12 12 11 13 14 12 12 12 Weight-averagemolecular weight of 12,300 11,900 12,600 12,600 12,100 11,300 11,50011,600 polystyrene block of block copolymer composed of components (a),(b) and (c) Weight-average molecular weight of block 343,000 339,000438,000 312,000 276,000 414,000 409,000 405,000 copolymer composed ofcomponents (a), (b) and (c) (a) Diblock polymer (wt. %) 17.6 20.3 7.327.8 32.3 4.1 3.4 11.3 (b) Four-branch polymer (wt. %) 43.7 61.1 72.452.3 60.6 72.4 62.9 83.2 Two-branch polymer (wt. %) 1.7 0 0 0.8 0 0 1.30 Three-branch polymer (wt. %) 37.0 18.6 20.3 19.1 7.1 23.5 32.4 5.5 (c)Two-branch polymer + three-branch 38.7 18.6 20.3 19.9 7.1 23.5 33.7 5.5polymer (wt. %) Formulation of pressure-sensitive adhesive compositionBlock copolymer (parts by weight) 100 100 100 100 100 100 100 100Tackifier resin (parts by weight) Aliphatic (C5 type) petroleum resin77.8 77.8 0 0 85.2 77.8 77.8 77.8 Softening point 96° C. 95° C. 70° C.96° C. 96° C. 96° C. Copolymerized (C5-C9 type) 85.2 85.2 petroleumresin Softening point 80° C. 80° C. Copolymerized petroleum resin ASoftening point Copolymerized petroleum resin B Softening pointNaphthenic process oil (parts by 7.4 7.4 0 0 0 7.4 7.4 7.4 weight)Antioxidant (parts by weight) 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92Characteristics of pressure-sensitive adhesive composition Holding power(min.) 1230 1520 1680 1470 1120 4870 4220 2670 Paper peeling test 3.53.5 4 3.5 3 3.5 3.5 3.5 Example Comparative Example 9 10 11 1 2 3 4 5Block copolymer composition I C C J K L M N Styrene content (wt. %) 1911 11 12 30 19 14 21 Weight-average molecular weight of 12,800 12,60012,600 12,100 15,700 11,500 10,700 17,800 polystyrene block of blockcopolymer composed of components (a), (b) and (c) Weight-averagemolecular weight of 327,000 438,000 438,000 298,000 167,000 196,000237,000 355,000 block copolymer composed of components (a), (b) and (c)(a) Diblock polymer (wt. %) 3.8 7.3 7.3 21.7 28.1 30.1 12.4 6.7 (b)Four-branch polymer (wt. %) 74.7 72.4 72.4 7.9 0 15.3 3.5 70.5Two-branch polymer (wt. %) 0 0 0 3.8 2.2 0 59.7 0 Three-branch polymer(wt. %) 21.5 20.3 20.3 57.7 69.7 54.6 24.4 22.8 (c) Two-branch polymer +three-branch 21.5 20.3 20.3 61.5 71.9 54.6 84.1 22.8 polymer (wt. %)Formulation of pressure-sensitive adhesive composition Block copolymer(parts by weight) — 100 100 100 100 100 100 100 Tackifier resin (partsby weight) Aliphatic (C5 type) petroleum resin 77.8 100 100 77.8 100Softening point 96° C. 96° C. 96° C. 96° C. 96° C. Copolymerized (C5-C9type) petroleum resin Softening point Copolymerized petroleum resin A77.8 Softening point 95° C. Copolymerized petroleum resin B 85.2Softening point 80° C. Naphthenic process oil (parts by 7.4 0 7.4 38.138.1 7.4 38.1 weight) Antioxidant (parts by weight) 0.92 0.92 0.92 0.920.92 0.92 0.92 Characteristics of pressure-sensitive adhesivecomposition Holding power (min.) — 2010 1750 770 850 320 640 470 Paperpeeling test — 4.5 4.5 3.5 1 2 3 1

When the makeup of the block copolymer compositions of Examples 1-11 iscompared with that of the block copolymer compositions of ComparativeExamples 1-4, the content of a diblock polymer constituting component(a) is in the range of 1 to 34% by weight for all compositions, but thetwo groups of compositions differ in the contents of components (b) and(c). Specifically, in the compositions of Examples 1-11, the content ofa four-branch polymer constituting component (b) is in the range of 34to 99% by weight, and the total content of two-branch and three-branchpolymers constituting component (c) is not greater than 50% by weightand less than the content of component (b). However, in the compositionsof Comparative Examples 1-4, the content of a four-branch polymerconstituting component (b) is less than 34% by weight, and the contentof component (c) is greater than 50% by weight. Owing to this differencein makeup, the pressure-sensitive adhesive compositions of Examples 1-11have excellent holding power as evidenced by values of greater than1,100 minutes, and exhibit excellent paper peeling test results asevidenced by scores of 3 or greater, whereas the pressure-sensitiveadhesive compositions of Comparative Examples 1-4 have a holding powervalue of not greater than 850 minutes and show a paper peeling testscore of not greater than 3.5. Among the pressure-sensitive adhesivecompositions of Comparative Examples 1-4, that of Comparative Examples 2has the highest holding power value (850 minutes), but shows a paperpeeling test score of 1. On the other hand, those of ComparativeExamples 1 and 4 show relatively high paper peeling test scores (3.5 and3), but have holding power values of 770 and 640 which are lower ascompared with Examples 1-11. From these facts, it can be seen that thepressure-sensitive adhesive compositions of the present invention haveexcellent holding power and exhibit excellent paper peeling testresults.

Moreover, it can also be seen that the pressure-sensitive adhesivecompositions of Examples 1-11 are more excellent in holding power andpaper peeling test results, as compared with the pressure-sensitiveadhesive composition of Comparative Example 5 in which polybutadiene wasintroduced at one end of a diblock copolymer.

What is claimed is:
 1. A mixture of poly(aromatic vinyl)/polyisopreneblock copolymers comprising 1 to 34% by weight of a diblock polymerrepresented by the following General Formula 1 and referred to ascomponent (a), 34 to 99% by weight of a four-branch polymer representedby the following General Formula 2 and referred to as component (b), and0 to 50% by weight of at least one branched polymer selected from thegroup consisting of a two-branch polymer represented by the followingGeneral Formula 3 and a three-branch polymer represented by thefollowing General Formula 4, and preferred to as component (c), whereinthe poly(aromatic vinyl)/polyisoprene block copolymer composed ofcomponent (a), component (b) and component (c) and contained in thepoly(aromatic vinyl)/polyisoprene block copolymer composition has aweight-average molecular weight (Mw) of 260,000 to 500,000, and thepoly(aromatic vinyl)/polyisoprene block copolymer composed of component(a), component (b) and component (c) and contained in the poly(aromaticvinyl)/polyisoprene block copolymer composition has a poly(aromaticvinyl) block content of 5 to 24% by weight: A¹-B¹  General Formula 1:wherein A¹ is a poly(aromatic vinyl) block having a weight-averagemolecular weight (Mw) of 9,000 to 20,000, and B¹ is a polyisopreneblock; (A²-B²)₄X²  General Formula 2: wherein A² is a poly(aromaticvinyl) block having a weight-average molecular weight (Mw) of 9,000 to20,000, B² is a polyisoprene block, and X² is a residue of a couplingagent having a functionality of 4 or higher; (A³-B³)₂X³  General Formula3: wherein A³ is a poly(aromatic vinyl) block, B³ is a polyisopreneblock, and X³ is a residue of a coupling agent having a functionality of2 or higher; (A⁴-B⁴)₃X⁴  General Formula 4: wherein A⁴ is apoly(aromatic vinyl) block, B⁴ is a polyisoprene block, and X⁴ is aresidue of a coupling agent having a functionality of 3 or higher, withthe proviso that the amount of the two-branch polymer is not greaterthan 10% by weight.
 2. A mixture of poly(aromatic vinyl)/polyisopreneblock copolymers as claimed in claim 1 wherein the poly(aromaticvinyl)/polyisoprene block copolymer composed of component (a), component(b) and component (c) has a weight-average molecular weight (Mw) of260,000 to 470,000.
 3. A mixture of poly(aromatic vinyl)/polyisopreneblock copolymers as claimed in claim 1 wherein the content of component(a) is in the range of 1 to 30% by weight.
 4. A mixture of poly(aromaticvinyl)/polyisoprene block copolymers as claimed in claim 1 wherein thecontent of component (b) is in the range of 45 to 99% by weight.
 5. Amixture of poly(aromatic vinyl)/polyisoprene block copolymers as claimedin claim 4, wherein the content of component (b) based on the totalamount of components other than the diblock polymer constitutingcomponent (a) is not less than 50% by weight.
 6. A mixture ofpoly(aromatic vinyl)/polyisoprene block copolymers as claimed in claim 1wherein the content of component (c) is in the range of 0 to 40% byweight.
 7. A mixture of poly(aromatic vinyl)/polyisoprene blockcopolymers as claimed in claim 1 wherein the poly(aromatic vinyl) blocksA¹, A², A³ and A⁴ have a weight-average molecular weight of 9,500 to17,000.
 8. (Amended) A mixture of poly(aromatic vinyl)/polyisopreneblock copolymers as claimed in claim 1 wherein the poly(aromaticvinyl)/polyisoprene block copolymer composed of component (a), component(b) and component (c) has a poly(aromatic vinyl) block content of 10 to18% by weight.
 9. A pressure-sensitive adhesive composition comprising amixture of poly(aromatic vinyl)/polyisoprene block copolymers as claimedin claim 1, and a tackifier resin.
 10. A process for producing a mixtureof poly(aromatic vinyl)/polyisoprene block copolymers as claimed inclaim 1, the process comprising the steps of (1) forming a poly(aromaticvinyl) block A having an active polymerizing end, (2) adding isoprene soas to form an A-B block copolymer in which a polyisoprene block B havingan active polymerizing end is directly joined to the poly(aromaticvinyl) block A, and (3) reacting the A-B block copolymer with a couplingagent having a functionality of 4 or higher in the presence of acoupling accelerator so as to convert 34 to 99% by weight of the A-Bblock copolymer into the four-branch polymer represented by GeneralFormula
 2. 11. A process as claimed in claim 10 wherein, in the step (3)of reaoting the A-B block copolymer with a coupling agent having afunctionality of 4 or higher in the presence of a coupling acceleratorso as to component 34 to 99% by weight of the A-B block copolymer intothe four-branch polymer represented by General Formula 6, the couplingagent is added in two or more divided portions.
 12. A process as claimedin claim 11, wherein the coupling agent is added in two dividedportions.
 13. A process as claimed in claim 11 wherein the couplingagent is added in two divided portions in which the first portioncomprises 40 to 90 mole % of the total amount of coupling agent used andthe second portion comprises 10 to 60 mole % of the total amount ofcoupling agent used.
 14. A process as claimed in claim 13, wherein thefirst portion comprises 50-90 mole % of the total amount of couplingagent used and the second portion comprises 10-50 mole % of the totalamount of coupling agent used.
 15. A process for producing a mixture ofpoly(aromatic vinyl)/polyisoprene block copolymers comprising 1 to 34%by weight of a diblock polymer represented by the following GeneralFormula 5 and referred to as component (X), 34 to 99% by weight of afour-branch polymer represented by the following General Formula 6 andreferred to as component (Y), and 0 to 50% by weight of at least onebranched polymer selected from the group consisting of a two-branchpolymer represented by the following General Formula 7 and athree-branch polymer represented by the following General Formula 8, andreferred to as component (Z), the process comprising the steps of (1)bringing an organolithium initiator into contact with an aromatic vinylmonomer to form a poly(aromatic vinyl) block A having an activepolymerizing end, (2) adding isoprene so as to form an A-B blockcopolymer in which a polyisoprene block B having an active polymerizingend is directly joined to the poly(aromatic vinyl) block A, and (3)reacting the A-B block copolymer with a halogenated silane couplingagent having a functionality of 4 or higher in the presence of acoupling accelerator so as to convert 34 to 99% by weight of the A-Bblock copolymer into the four-branch polymer represented by GeneralFormula 6: A⁵-B⁵  General Formula 5: wherein A⁶ is a poly(aromaticvinyl) block, and B⁵ is a polyisoprene block; (A⁶-B⁶)₄X⁶  GeneralFormula 6: wherein A⁶ is a poly(aromatic vinyl) block, B⁶ is apolyisoprene block, and X⁶ is a residue of a coupling agent having afunctionality of 4 or higher; (A⁷-B⁷)₂X⁷  General Formula 7: wherein A⁷is a poly(aromatic vinyl) block, B⁷ is a polyisoprene block, and X⁷ is aresidue of a coupling agent having a functionality of 2 or higher;(A⁸-B⁸)₃X⁸  General Formula 8: wherein A⁸ is a poly(aromatic vinyl)block, B⁸ is a polyisoprene block, and X⁸ is a residue of a couplingagent having a functionality of 3 or higher, with the proviso that theamount of the two-branch polymer is not greater than 10% by weight. 16.A process as claimed in claim 15, wherein the coupling accelerator is atleast one compound selected from the group consisting of an aromaticether, an aliphatic ether, a tertiary monoamine and a tertiarypolyamine.
 17. A process as claimed in claim 15 wherein the tertiarypolyamine is N,N,N′,N′-tetramethylethylenediamine orN,N,N′,N′-tetraethylethylenediamine.
 18. A process as claimed in claim17 wherein the total amount of tertiary polyamine used in step (1)through step (3) is in the range of 0.02 to 1 mole, per mole of theorganolithium initiator.
 19. A process as claimed in claim 17 whereinthe coupling accelerator is added in step (1) and further added afterstep (2).
 20. A process as claimed in claim 19 wherein the amount oftertiary polyamine further added after step (2) is in the range of 0 to0.4 mole, per mole of the organolithium initiator.
 21. A process asclaimed in claim 15 wherein the tetrafunctional silane compound is atleast one compound selected from the group consisting oftetrachlorosilane, tetrabromosilane, tetramethoxysilane andtetraethoxysilane.
 22. A process as claimed in claim 15 wherein thetotal amount of coupling agent used is in the range of 0.05 to 0.4 mole,per mole of the organolithium initiator.
 23. A process as claimed inclaim 22, wherein the total amount of coupling agent used is in therange of 0.085 to 0.3 mole, per mole of the organolithium initiator.